This invention relates to the reduction of brightness reversion in paper products, particularly those made from high-yield pulps and to compositions for achieving same.
A "high-yield" pulp is a pulp in which the papermaking fibres are separated by means which are mainly mechanical action (optionally with some chemical treatment). As the name indicates, there is relatively little waste with such a pulp, in contrast to the so-called "chemical" pulps where the fibre separation is achieved by purely chemical means. Paper made from high-yield pulp (for example, groundwood pulp, refiner pulp, thermomechanical pulp (TMP), and chemithermomechanical pulp (CTMP)) is considerably cheaper than that made from chemical pulp, even when the high-yield pulp is bleached. The problem which has restricted the use of high-yield pulps to non-archival uses such as newsprint and corrugated paper is the tendency of such paper to undergo brightness reversion (yellowig) with time, this being believed to be caused by the instability to ultraviolet light of the lignin component of high-yield pulps.